This invention relates to a control valve of the type utilized in a hydrostatic steering system.
Control valves of the foregoing type having two relatively rotatable slide valve elements that are axially biased in one direction by fluid pressure, are well known. In such control valves, the radially outer one of the two slide valve elements is torsionally connected to the rotor of a metering hand pump while the radially inner valve element is connected to a steering spindle. The internal space of the inner slide valve element and the axial ends of the two slide valve elements facing the metering hand pump, are acted upon by the pressure fluid of a pressure source. The opposite axial ends of the slide valve elements, face an abutment in the housing and are free of axial pressure. Consequently, a substantial axial force acts on the two slide valve elements tending to displace the valve elements toward the housing abutment.
In prior control valve assemblies, the two slide valve elements are provided with an axial thrust bearing on the housing abutment in the form of an annular disk to reduce high frictional resistance to rotation of the slide valve elements. Such bearing support for the valve elements while inadequate, is apparently tolerated because of the difficulties in providing an anti-friction type of bearing with a relatively small diameter. Anti-friction bearings capable of sustaining the required loading are not available for such small diameter installations. Any enlargement of the bearing diameters cannot be tolerated because the generated surface of both slide elements that determine the bearing diameters, are sealing surfaces. Any enlargement of such sealing surfaces, even in the order of 5 mm, would result in substantial fluid leakage.
It is therefore an important object of the present invention to provide anti-friction bearings for both rotatable slide valve elements of a hydrostatic steering control valve assembly for the absorption of axial thrust loads without diametrical enlargement of the sealing surfaces of the slide valve elements.